1. Field of the Invention
The present invention relates to a single-layer electrophotographic photoconductor containing a photosensitive layer which contains at least a certain electron transporting material and a hole transporting material.
The present invention also relates to a positively charged electrophotographic photoconductor, a positively charged image forming apparatus and a process cartridge using the positively charged electrophotographic photoconductor which contains a single photosensitive layer containing a combination of a certain charge transporting material and an organic sulfur antioxidant, and does not generate an abnormal image such as afterimage even after repeated use.
2. Description of the Related Art
In recent years, developments of information processing system using electrophotographic system are remarkable. Particularly the optical printers in which information is converted to digital signals to be recorded by light have been notably improved in terms of printing quality and reliability. The digital recording technique of this type is also applied to general copiers as well as printers and so-called “digital copiers” have been developed. Moreover, the demand for the copiers in which the conventional copiers which have been provided with the digital recording technique is expected to increase more in the future because of additional various information processing functions. Furthermore, developments of digital color printers for outputting color images and documents are drastically advancing with popularization and upgrade of personal computers.
The electrophotographic photoconductor used in the image forming apparatuses as described above can be classified broadly into an organic photoconductor and an inorganic photoconductor. The organic photoconductors are being widely used recently because it can be manufactured easily and inexpensively as compared with the conventional inorganic photoconductors, and there is a lot of flexibility in functional designs because of various choices for photoconductor materials including a charge transporting material, a charge generating material, a binder resin, and the like.
Examples of the organic photoconductors include a single-layer photoconductor in which the charge transporting material (a hole transporting material and an electron transporting material) is dispersed together with the charge generating material in a photosensitive layer, and a multilayer photoconductor in which a charge generating layer containing the charge generating material, and the charge transporting layer containing the charge transporting material are layered.
The multilayer photoconductors are mostly negatively charged, and the positively charged multilayer photoconductor has not been achieved in practical use. This is because the electron transporting material which is excellent in electron transporting ability, less toxicity, and has high compatibility with the binder resin has not been achieved yet.
On the other hand, the single-layer photoconductor in which the charge generating material and the charge transporting material are contained in a single-layer photosensitive layer has been drawing attention recently for the following reasons: capable of manufacturing by a simple manufacturing process; improvement of optical property due to fewer layer interfaces; capable of positively charged with excellent in uniform charge property and the small amount of generated ozone due to having sensitivity of both positive and negative polarity by containing the electron transporting material and the hole transporting material.
In the single-layer photoconductor, the charge generating material is generally contained throughout the photosensitive layer, thus, charge is basically generated throughout the layer. A semiconductor laser (LD) and a light emitting diode (LED) are generally used as the light sources for exposing in the digital image forming apparatus of recent years, and its wavelength is mainly near infrared of approximately 680 nm to 830 nm. With the light source of the long wavelength range, and the light is penetrated into the depth of the photosensitive layer, and hole-electron pairs are formed throughout the layer. This may easily interfere the mobility of the hole and the electron due to the difference between the mobility of the hole and that of the electron, structural defect, and recombination.
Therefore the single-layer photoconductor easily invites less sensitivity and rise of residual potential after repeated use, and may easily generate an abnormal image so-called “afterimage”.
An image forming apparatus using an electrophotographic system generally forms an image by charging a photoconductor (charging step), exposing imagewise to form a latent electrostatic image (exposing step), developing the latent electrostatic image by applying a developing bias voltage to form a toner image (developing step), transferring the toner image on a transfer paper (transferring step), and fixing. A residual toner on the photoconductor is cleaned by an urethane blade and the like (cleaning step), and residual potential on the photoconductor is remove by LED and the like (charge removing step).
The afterimage is generated such that carriers accumulate in the exposed part in the exposing step, and the effect of exposing is remained even after the charge removing step, which is exposed again with generating potential difference in the next charging step. Thus the electric potential in the exposed part after exposing is lower than the circumference, and then the afterimage is generated as nonuniform density on an image.
Japanese Patent Application Laid-Open (JP-A) Nos. 8-328275, 7-64301, 9-281729, 6-130688, and 9-151157 disclose the conventional single-layer photoconductors. Their properties are not satisfied because the properties are significantly lowered, and afterimage is generated after repeated use.
In a negatively charged photoconductor, corona discharge is unstable as compared to in a positively charged photoconductor, and ozone and nitrogen oxides are generated. These are adsorbed to the surface of the photoconductor and easily cause physical and chemical degradation, moreover, adversely affect to the environment. Therefore, the positively charged photoconductor is more widely used than the negatively charged photoconductor due to greater flexibility in use condition.
The single-layer photoconductor is exemplified as the positively charged photoconductor. The single-layer photoconductor mainly contains both of the electron transporting material and the hole transporting material as a charge transporting material. Thus, the single-layer photoconductor has a sensitivity of positive and negative polarity. However, most of the single-layer photoconductor is positively charged because of the higher sensitivity in positive charge due to the lower electron transporting ability of the electron transporting material, and benefit from the positive charge.
The conventional single-layer photoconductors disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 8-328275, 7-64301, 9-281729, 6-130688, and 9-151157. These single-layer organic photoconductors have problems inherent in the single-layer photoconductor such that higher residual potential, and greater fluctuation in charged electric potential due to repeated electrostatic fatigue and in electric potential after exposing as compared to a separated-function multilayer photoconductor.
To solve the problem of the single-layer photoconductor, in recent years, a novel electron transporting material has been developed. Particularly, International Publication No. WO 2005/092901 discloses a tetracarboxylic acid derivative, and a naphthalenecarboxylic acid derivative which have excellent electron transporting ability, thus the problem of the conventional single-layer photoconductor can be solved, and electrostatic property is greatly improved.
An electron transporting material expressed by the General Formula (1) in the present invention which is disclosed in International Publication No. WO 2005/092901 has an excellent electron transporting ability. The single-layer photoconductor using the electron transporting material is an excellent single-layer photoconductor because it has high sensitivity, and is less decrease of sensitivity after repeated use. However, there is a problem that the single-layer photoconductor using the electron transporting material has low charge property the same as the conventional single-layer photoconductor. The single-layer photoconductor also has low charge stability, thus the charged electric potential is lowered after repeated use, and abnormal images such as background smear and fog may easily generate.
Moreover, the single-layer photoconductor has a problem that an afterimage (memory image) is easily generated. In the reversal developing system which is a mainstream system in digital image forming apparatus of recent years, the photoconductor is charged, an image part is exposed, the part of the lower surface potential of the photoconductor is developed using the toner having the same polarity with the photoconductor, and bias voltage of reversal polarity is applied to the photoconductor so as to transfer a toner image to a transfer medium in the transferring step. In the transferring step, the surface potential of the image part is reversely charged from main potential of the photoconductor in the transferring step, because the reversal bias is applied to the image part in the condition of low surface potential. The single-layer photoconductor has the sensitivity of both positive and negative polarity because it contains the electron transporting material and the hole transporting material. When the image part is reversely charged, the polarity can be partially cancelled by charge removal by light, but can not completely cancelled. Thus potential difference remains. The photoconductor having enough charging ability can cancel the potential difference in the next charging step, and can be uniformly charged. The photoconductor having lower charging ability cannot cancel the potential difference in the next charging step, and a record of the prior image remains in the next image.
The single-layer photoconductor has low charge stability, and easily generates the afterimage after repeated use.
The electron transporting material expressed by the General Formula (1) may significantly improves the sensitive property of the single-layer photoconductor, however, it has a problem in charging ability the same as the conventional single-layer photoconductor, and an afterimage is easily generated after repeated use. Therefore, the sufficient result has not been obtained at present.